1. Field of the Invention
The present invention relates to an apparatus and method for liquefying solid materials. More particularly, the invention relates to an on-demand in-line liquefier of solid chunks of aqueous gelatin solutions.
2. Description of the Related Art
Various types of heating systems which heat solid materials up to their liquefying temperature are known. Upon reaching the liquefying temperature, the solid material is transformed into a liquid and subsequently provided to an apparatus where it is dispensed. Examples of such liquefying systems are U.S. Pat. No. 4,641,764 and U.S. Pat. No. 4,505,669, which are both concerned with heating and liquefying high viscosity hot-melt material, such as thermoplastics, particularly sealants and adhesives. However, in each of these systems, the liquefied material is immediately drawn off and used. That is, when a hot-melt adhesive is produced, it is typically pumped to an applicator and immediately applied to the items which are to be bonded. Accordingly, neither of these systems is concerned with the temporary storage of the liquefied material and the requirements of thermal maintenance associated therewith.
Moreover, as the solid material melts in each of the aforementioned systems, there is a tendency for the liquefied material to foam up around the heating elements due to the entrained air contained in the liquefied material. This foaming material often causes a "skin" or "film" to form on the heating elements which continues to build up and results in a decrease in heat transfer and a corresponding decrease in the liquefying rate. Ultimately, the decrease in heat transfer can be so large that liquefaction of the solid material becomes impossible. The changing liquefying rate is unacceptable for continuous operations where the liquefied material is being subsequently mixed proportionally and in-line with other materials. This is particularly true in the photographic industry where solid chunks of aqueous gelatin solutions are melted and automatically fed in-line to mix with other chemical compositions to produce high quality photographic products which are chemically consistent from batch to batch.
An additional problem with conventional liquefying apparatus concerns the design of the heating elements. Often, it is the case that the spacing between the heating elements is too small. When the solid material melts, puddles of liquefied material are created at certain locations on the heating elements. These puddles will foam up on the heating element as discussed above, continually reducing the liquefying rate until eventually liquefaction is impossible. On the other hand, if the spacing between the heating elements is too large, solid chunks of material will pass through the heating elements without being liquefied.
U.S. Pat. No. 3,395,694 discloses another example of an adhesive heating apparatus. However, in this apparatus, while the liquefied adhesive is temporarily stored in a sump, a separate heating system is required to maintain the temperature distribution within the sump. This additional heating system makes the apparatus complex and costly.
Still another problem with conventional liquefying systems is exemplified in United Kingdom Patent No. 1,501,515 which discloses a process and apparatus for the production of photographic materials. In the melting zone of the disclosed apparatus, the temperature of a melting grid is controlled by introducing saturated steam into the grid or by preventing saturated steam from entering the grid. However, in this apparatus the temperature of the saturated steam in the grid is allowed to vary. Thus, when saturated steam is required in the melting grid to perform the melting operation, there can be a delay in the melting process associated with the time between the initial entry of saturated steam into the grid until the required melting temperature of the steam within the grid is met. This delay makes it difficult to control the rate of the liquefaction process.
Thus, there has been a need for a passive liquefier which can be used in the photographic industry for liquefying solid chunks of aqueous solution in a simple and efficient manner, such that the liquefaction rate will be uniform, and the temperature distribution of the liquefied gelatin solution that is temporarily stored in a reservoir can be uniformly maintained in a simple and cost effective manner. Additionally, there has been a need for a passive liquefier which ensures that the heating medium provided to a heating element is provided at a substantially constant pressure and temperature to preclude any delay in initiating the melting process. Moreover, it is desirable to provide a method for accomplishing the efficient liquefaction of solid materials in a simple manner.